Wireless or mobile (cellular) communications networks in which a mobile terminal (UE, such as a mobile handset) communicates via a radio link to a network of base stations (referred to variously as E-UTRAN Node B, eNodeBs eNBs) or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations. Base stations serve one or more “cells”, which define respective geographic areas of radio access coverage. Cells in turn are typically split into sectors. The initial deployments of systems using analogue signaling were superseded by Second Generation (2G) digital systems such as Global System for Mobile communications (GSM), which typically use a radio access technology known as GSM Enhanced Data rates for GSM Evolution Radio Access Network (GERAN), combined with an improved core network.
Second generation systems have themselves been largely replaced by or augmented by Third Generation (3G) digital systems such as the Universal Mobile Telecommunications System (UMTS), which uses a Universal Terrestrial Radio Access Network (UTRAN) radio access technology and a similar core network to GSM. UMTS is specified in standards produced by 3GPP. Third generation standards provide for a greater throughput of data than is provided by second generation systems. This trend is continued with the move towards Fourth Generation (4G) systems.
3GPP design, specify and standardize technologies for mobile wireless communications networks. Specifically, 3GPP produces a series of Technical Reports (TR) and Technical Specifications (TS) that define 3GPP technologies. The focus of 3GPP is currently the specification of standards beyond 3G, and in particular an Evolved Packet System (EPS) offering enhancements over 3G networks, including higher data rates. The set of specifications for the EPS comprises two work items: Systems Architecture Evolution (SAE, concerning the core network) and LTE concerning the air interface. The first set of EPS specifications were released as 3GPP Release 8 in December 2008. LTE uses an improved radio access technology known as Evolved UTRAN (E-UTRAN), which offers potentially greater capacity and additional features compared with previous standards. SAE provides an improved core network technology referred to as the Evolved Packet Core (EPC). Despite LTE strictly referring only to the air interface, LTE is commonly used to refer to the whole of the EPS, including by 3GPP themselves. LTE is used in this sense in the remainder of this specification, including when referring to LTE enhancements, such as LTE Advanced. LTE is an evolution of UMTS and shares certain high level components and protocols with UMTS. LTE Advanced offers still higher data rates compared to LTE and is defined by 3GPP standards releases from 3GPP Release 10 up to and including 3GPP Release 12. LTE Advanced is considered to be a 4G mobile communication system by the International Telecommunication Union (ITU).
Multicast and broadcast user services, where the services can be supplied to more than one user simultaneously, are facilitated by Multimedia Broadcast Multicast Services, MBMS, in LTE. MBMS was introduced as a component of E-UTRAN where the standard specified support for the delivery of broadcast services such as the transmission of television services and the transmission of multimedia content (e.g. audio, video, still images, text, etc.). While MBMS is a user service that employs a broadcast mechanism at the radio interface, the provision of security features can ensure that only particular UEs receive a service (i.e. UEs belonging to a multicast group). An area in which data of a specified MBMS service is transmitted (when such data is scheduled for transmission) is referred to as an MBMS service area: note that a single MBMS service area corresponds to an aggregation of one or more discrete geographic areas.
MBMS has been developed from its E-UTRAN/UMTS roots to allow the synchronization of the delivery of multicast user services across a plurality of cells. The introduction of MBMS over Single Frequency Network (MBSFN) means that it is possible to simultaneously transmit identical data streams across more than one cell using the same radio frequency (rf) carrier: with MBSFN, identical content (control and payload) is transmitted at substantially the same time across the or each participating cell. MBSFN has been configured to make exclusive use of the radio resources for certain subframes within each radio frame.
MBSFN relies upon synchronization between an MBMS service center and each base station providing a participating cell and a SYNC protocol has been specified (in 3GPP TS 25.446). A group of base stations can be synchronized to define an MBSFN Synchronization Area: each MBSFN Synchronization Area can support one or more MBSFN Areas (i.e. groups of cells within the MBSFN Synchronization Area that, together, provide an MBSFN transmission for a given MBMS service). It is possible that a given cell can provide support for up to eight different MBSFN Areas allowing the concurrent broadcast of a plurality of MBMS services. Furthermore a single MBSFN area can also support more than one service.
From 3GPP Release 10, a counting procedure has been introduced in MBMS to allow the network to quantify the number of active UEs in each cell that are receiving (or interested in receiving) a given MBMS service via a multicast bearer, MRB, (described in 3GPP TS 36.443). Further enhancements were introduced in Release 11 to accommodate improved service continuity for MBMS services.
Since MBMS is a multicast/broadcast system that provides the same information to multiple users, the data rate that is possible in any MBSFN area is limited by the radio link conditions for the MBSFN transmission at the worst placed UE, as no user-specific adaptation of transmission parameters is available.
In cellular networks generally, there is a constant effort to optimize the radio network while increasing data rate. In the initial radio network configuration or optimization stage, a base station or a base station controller should collect (i.e. measure) radio environment information related to its own cell coverage. The measurement of radio environment information assists in network planning and accurate empirical modelling of coverage. Typically, operators of mobile networks have driven (or otherwise transported) vehicles equipped with radio measurement apparatus over geographical areas of interest while performing the measurement task repeatedly. The measurement result is used to configure the system parameters of the base stations or base station controllers, even to identify potential sites for new base station installations. Use of vehicles in this manner is known as “drive testing”. While road (or waterway) systems often coincide with population centers, this has meant that network planning has been incomplete and/or potentially biased by accessibility. Furthermore, as such vehicles need suitably trained personnel and can only occupy a limited number of locations over a measurement period, drive testing is considered to be comparatively time consuming and expensive: drive testing increases total costs and time of the radio network optimization and maintenance.
Study on minimization of drive tests and enhancement of radio environment analysis process and manual configuration was conducted under the name of Minimization of Drive Test (MDT) and the 3GPP RAN2 Working Group introduced support for Minimization of Drive Tests in release 10 of the 3GPP standards. This introduced a framework governing the provision of measurement reports by the UEs of consenting subscribers; discussed in detail in technical specification: 3GPP TS 37.320: “UTRA and E-UTRA; Radio measurement collection for Minimization of Drive Tests (MDT); Overall description; Stage 2”. The MDT framework allows the operator to gather measurement information similar to that obtained by drive testing from UEs in the relevant geographic area(s).
To allow the network operator to optimize network provision of MBMS services and to ensure that the or each service can be received at an acceptable Quality of Service within the entire service area, it is therefore helpful to model the experience of UEs in the network using empirical data.
The E-UTRA specifications include two mechanisms (also referred to as “modes”) by which UEs can provide measurement results to the network: Logged MDT and Immediate MDT.
In Immediate MDT, a UE in connected mode can be configured to perform measurements and to report the results either periodically or when a certain event condition occurs.
Alternatively, in Logged MDT, a UE can be configured to log available results of measurements performed in while in idle mode, which the network can retrieve at a later point in time.
By contrast with drive testing, in MDT, User Equipment (UE) measures radio channels and reports the radio channel measurement information to an evolved Node B (eNB). The report may be immediate or deferred: deferred delivery may occur at predetermined intervals, immediately in response to a specific event, or after a predetermined time has elapsed from the time when the radio channel measurement information has been recorded. The process in which the UE transmits the radio channel measurement information and other supplementary information to the eNB is called MDT measurement information reporting. In certain embodiments, reporting may be delayed depending upon the state of connection between the UE and the eNB, so that the UE reports the channel measurement result after the completion of channel measurement provided the communication is available with the eNB (i.e. the UE is in connected mode) but delays the report while the communication is not available with the eNB (i.e. the UE is in idle state) until the connection to the eNB is recovered.
The eNB uses the MDT measurement information received from the UE for cell coverage optimization. RAN procedures are generally specified in a manner agnostic of the purpose for which E-UTRAN applies them (i.e. the usage is up to E-UTRAN implementation). For connected mode, the existing signaling procedures were largely reused with minimal extension, while for idle mode all procedures for supporting measurement logging and their retrieval were introduced from scratch.
To obtain measurement data, two categories of MDT operation are envisaged: an area based MDT and a signaling based MDT. In the former, the measurement data is gathered from all UEs of consenting subscribers within a predetermined geographical area (defined using a cell list, tracking area etc.). In the latter, specific UEs are addressed (using a unique identifier such as an IMSI, IMEI etc.) and requested to signal measurement results.
While the MDT framework is specified for E-UTRAN, there is no facility that would permit a network operator to gather any MBMS measurement information from UEs that would assist in ensuring that the service can be received properly within one or more MBMS service areas.